[Ca_(24)Al_(28)O_(64)]~(4+):4e~-电子化合物的制备及其电输运特性

金属氧化物电子化合物Ca_(24)Al_(28)O_(64)]~(4+):4e~-(C12A7:e~-)因其天然的纳米尺度笼腔结构带来的新奇物理化学特性而在阴极电子源材料、超导和电化学反应等领域有着独特的应用价值.本文系统研究了以CaCO_3和Al_2O_3粉末为原料,采用固相反应-放电等离子烧结-活性金属Ti还原相结合的方法制备C12A7:e~-的工艺条件及其电输运特性.实验结果表明:在封装石英管真空度为10~(-5)Pa,还原温度为1100?C,还原时间为10—30 h条件下,成功制得载流子浓度为约10~(18)—10~(20)cm~(-3)的C12A7:e~-块体材料.第一性原理计算得到的C12A7:e~-能带结构和态密度表明,笼腔内的O~(2-)完全被e-取代后,C12A7:e~-费米能级明显穿过笼腔导带,说明位于笼腔内自由运动的电子使C12A7从绝缘体转变成导体,同时费米面附近的笼腔电子易于从笼腔导带跃迁至框架导带,在电场或热场的作用下电子更容易逸出,这也是C12A7:e~-逸出功低的主要原因.

Fabrication and electrical transport characteristics of the polycrystalline Ca12Al14O33 electride

TheCa₂₄Al₂₈O₆₄]⁴⁺:4e^- (C12A7:e^-) electride composed of densely packed, subnanometer-sized cages. This unique structure makes it possess distinctive applications in fields of electronic emission, superconductor, electrochemical reaction. In this paper, we explore a new method to prepare the bulk of C12A7:e^- electride. The following areare systematically studied in this work. 1) the condition of preparing bulk of C12A7:e^- electride by solid reaction combining spark plasma sintering and reduction with Ti particles at high temperature, CaCO₃ and Al₂O₃ powders are used as raw materials; 2) the first principle calculations of band structure and density of states of the C12A7:e^- electride; 3) the analysis of the electrical transport properties of the C12A7:e^- electride. The bulk of C12A7:e^- electride is successfully prepared by this method, so the results show that the bulk of C12A7:e^- electrode with the electron concentration 10¹⁸-10²⁰ cm^-3 is synthesized at 1100 ℃ and a vacuum pressure of 10^-5 Pa for 10-30 h. In the process of Ti reduction, Ti particles become evaporated and deposit on the surface of C12A7, the free O^2- atom in the cages diffuse to the sample surface, the Ti vapor reacts with the O^2-, forming a loose TiO_x layer. In order to maintain electrical neutrality, the electrons of the free O^2- atom leave from the cages, forming the C12A7:e^- electride. In addition, the loose TiO_x layer also provides a channel for the diffusion of the O^2- atoms in the cage, ensuring the continuation of the reduction reaction. The calculated band structure and density of states of the bulk C12A7:e^- electride show that when electrons replace the O^2- atoms in the cage, the Fermi level of C12A7:e^- crosses over the cage conduction band (CCB). Thus the free movement of the electron is the main reason for the insulator C12A7 to convert into conductor C12A7:e^-. At the same time the electrons near the Fermi level in the cages are easy to jump from the CCB to the frame conduction band (FCB). Combination of the above experimental results suggests that the electrons in cages are easier to escape to vacuum under the action of electric field or thermal field, which is the main reason for low work function of C12A7:e^-. This way provides an new approach to the realization of the insulator C12A7 converting into C12A7:e^- electride. And the C12A7:e^- is a good electronic emission material due to low work function, low working temperature, and highly anti-poisoning ability, so this method of preparing bulk C12A7:e^- electride provides a good new way to synthesize a new electronic emission material.